1. Field of the Invention
The present invention relates generally to electronic device plastic mold package decapsulation, and more particularly to jet etching of such devices.
2. Description of the Prior Art
Decapsulation of integrated circuits (IC) in their plastic packages has become widely popular as a method incorporated in analyzing such circuits to ensure their reliability and to reverse engineer devices. One method of decapsulation involves a very risky process of pooling fuming nitric or sulfuric acid, and requires great skill in order to avoid damaging the samples to be analyzed.
Both dry and wet methods of decapsulation are conventionally used to open plastic mold packages. The wet method uses an etching process by acid or solvent. The dry method employs burning with high frequency-excited oxygen or gaseous FREON. The dry method has a much poorer processing capacity, and so is seldom used.
Conventional wet methods include one in which a device-under-test (DUT) is masked with adhesive tapes and soaked in fuming nitric or sulfuric acid heated to 60.degree.- C. Only the unmasked portions will be dissolved. Another wet method indents the device-under-test on its chip-side with an end mill, or similar machine, and heats it on a hot plate. Then fuming nitric or sulfuric acid is pooled into the indentations to commence a gradual etching in locations controlled by the indentations.
Red fuming nitric acid is reported by at least one industry equipment supplier to be thoroughly tested by the industry's leading failure analysis and reliability engineers. They have determined that red fuming nitric acid is a good etchant for decapsulating plastic mold IC packages having aluminum components. An alternate etchant, concentrated sulfuric acid, is claimed to cause damage to both the device-under-test and aluminum bonding pads. The present inventor, however, has not experienced such problems with aluminum corrosion when using sulfuric acid. When dispensed at a relatively low temperature, e.g., 65.degree. C., red fuming nitric acid reportedly forms an oxidation layer over any aluminum components, thereby protecting them from damage by corrosion. The present inventor prefers the use of concentrated sulfuric acid since it forms an oxidation layer on exposed aluminum at higher temperatures, e.g., 220.degree. C. Thus, faster decapsulation is possible, compared to fuming nitric acid.
A jet etch apparatus for decapsulation of molded devices is described in U.S. Pat. No. 4,344,809, issued Aug. 17, 1982, to the present inventor, Ben L. Wensink (Wensink '809). An etchant solution is drawn through an etching block and forms a jet spray that impinges on the device-under-test. A jet pump creates a suction that draws the etchant solution into the jet stream. The decapsulation process is monitored by
detecting a current flow between an electrode in contact with the etchant solution and the device. The imposed current serves to protect the aluminum against corrosion by cathodic action, and it assists in monitoring the progress of decapsulation.
A jet etch method for decapsulation of molded
devices is described in U.S. Pat. No. 4,384,917, issued May 24, 1983, also to the present inventor, Ben L. Wensink (Wensink '917).
The Wensink patents describe an etch head with a shallow recess with a nozzle opening in relatively close proximity to the device-under-test which is on top of the etch head. Fresh etchant solution from the nozzle reaches the device-under-test there and can etch the surface.
For the relatively large dual-in-line (DIP) plastic packages that were prevalent in the 1980's, such a construction was very sensible. A plastic DIP could be used to cover the recess opening in the etch head and thereby seal in the necessary vacuum within the recess to support decapsulation jet spraying of the etchant solution. But there has been a trend toward smaller and thinner packages that have connector prongs instead of pins. Such prongs include J-types that exit the packages along the side edges and then curl back underneath. Small outline packages (SOT) have very tiny side pins. The variability in packaging and reduction in sizes has created difficulties in using conventional decapsulation machines and methods. Such packages become mechanically fragile after decapsulation, since most of the plastic material along the top surface of the package is dissolved away. Subsequent handling of the decapsulation device can easily result in losing pins and the electrical destruction of the device.
Other modern package concepts provide the necessary electrical connections to the semiconductor device die on the inside through a matrix of small solder bumps, e.g., bump grid array (BGA), or by small solder columns, e.g., column grid array (CGA). Both such packaging concepts are strongly promoted, for example by IBM and Motorola. Surface mount technology (SMT) packages include the use of gull-wing pins, S-shaped pins that horizontally exit the package along the edges and then are bent down and out again to form a horizontal foot.
A need therefore exists for fixtures that can be used in a decapsulation process that do not require that a device be removed from the fixture for subsequent electrical characterization. The decapsulation hole that is made needs to be strictly defined to allow for a ring of material that will be left undisturbed and that will allow a degree of mechanical stability to remain with the device-under-test for further study.
More advanced fixturing for jet etch machines is required for the larger die-size to package-size ratios that exist in modern device packages. Less plastic material exists around a typical semiconductor device die, and accurate placement of the package with respect to the decapsulation hole defined by an etch plate is increasing in importance. By combining the etch plate with the fixture, this goal could be readily accomplished. In the prior art, the accuracy of decapsulation depends on the placement accuracy of the device with respect to the etch head of the machine.